A dental implant is an artificial prosthesis normally comprised of a single cylindrical component to replace the missing root structure of a natural tooth that has been lost. This single stage is inserted into a prepared hollowed out bone preparation (osteotomy) in the patient's jawbone (endosseous) and typically remains buried there for a period of time to allow for “osseo-integration” or the growth and adhesion of natural bone around the implant “root screw”, securing it in place. This cylindrical implant typically contains down its internal center a machined threaded internal hollow bore that allows the dental practitioner upon later surgical exposure of the head or top section of the cylindrical implant to screw into place a machined screw-in abutment (either with an integral screw on its inferior aspect or a separate connector screw which threads through a center hollow sleeve of the abutment). The head section of the implant is simply the top segment of the cylindrical implant form and is an integral part of it. The abutment, which extends into the oral cavity, is then utilized by the dentist to fabricate a single fixed prosthesis (crown).
There are several major drawbacks to this standard implant design. These drawbacks are derived from the fact that the standard implant design form is actually in very significant variance to the natural structural form of the roots of human teeth. There are different types of teeth in the humans, namely, the upper and lower incisors, canines (cuspids), premolars, and molars. These teeth differ to a significant degree in form from each other between the different categories, and they differ as well within each category depending on whether they are in the upper or lower jaws and which position they have in each jaw. These differences in form (and structure) apply not only to what is termed in dentistry as the crown portion of the teeth (the part of the tooth that is erupted into the mouth and visible to the eye) but extends as well to the forms of the root (s) portion (buried in the alveolar bone socket of the jaws) of these different categories of teeth in both the maxilla and mandible.
The mesial aspects (part of the root structure that is deep in the bone) of the natural roots of teeth are basically cylindrical or somewhat oval in cross-section. When one though observes in cross-section the natural form of the roots of teeth at the level of the transition of the tooth from its root segment to its crown segment (known as the root trunk) at the crest of the jawbones (this level is referred to in dentistry as the CEJ—cemento-enamel junction or the cervix of the tooth) one is immediately struck by the fact that in general most of the root forms of the root trunks in cross-section of the teeth are anything but cylindrical in shape or form (the standard dental implant form is cylindrical in cross-section along its entire length). Depending on the type of tooth in question, the natural root trunk form of the teeth in cross-section are in fact very oval at this level (at the cervix), either in a horizontal axis in relation to the crestal bone ridge of the jaw when one is referring to incisors, or oval in a vertical axis in relation to the crestal ridge when one is referring to the premolars, and quite rhomboid, oval or kidney shaped when one is referring to the molars. In addition, when one is referring to the molars, the natural teeth typically exhibit multiple roots (typically the molars are bi-rooted in the mandible and tri-rooted in the maxilla).
The standard dental implant design (endosseous) being cylindrical in form along its entire length including the head or top segment of the implant, and consisting of a very limited number of different sized single “root screw” cylinders takes none of the above mentioned natural variation of the roots of the different types of teeth (particularly the back teeth-molars) into account, both in the maxilla and the mandible.
Due to its cylindrical form along its entire length, the standard dental implant does not conform at the level of the crest of jawbone (level of the root trunk) to the natural oval, rhomboid or kidney-shape form of the roots of the natural teeth which sit in the bone (the head or coronal section of the standard implant is cylindrical in cross-section as well). This major discrepancy in the contour or emergence profile, as it is termed in dentistry, of the crown that is fixed upon the implant abutment (which of necessity must fit precisely into the head portion of the implant) in relation to the gums (as compared to the emergence profile of the natural crown of a tooth as it emerges from the natural root trunk of the tooth) results in large gaps or spaces between the implant crown and the teeth on either side of it and prevents the optimal formation of the interdental papilla (gum tissue between the teeth). With the posterior implant, the situation is very much analogous to a large ball sitting on top of a thin stick, where the ball is the crown and the standard implant is the stick. These large open areas or gaps allow for food debris, plaque, and pathogenic bacteria to accumulate between the implant crown and the natural teeth adjacent to it, making these areas very difficult for the patient to keep clean and requiring the patient to use special cleaning implements to try and maintain them free of food debris and plaque. In many cases this situation over the long-term results in poor health of the gums, causing periodontal (gum) disease of the adjacent teeth as well as documented cases of implant failure due to crestal bone resorbtion.
Additionally, as was previously mentioned, standard implants on the market consist of a single cylindrical “root screw” form or stage that is buried into the alveolus (jawbone) to replace the natural root of the missing teeth. A second stage abutment is later screwed into the “root screw” (the abutment sits above the bone in the mouth) and a crown is made to sit on top of the abutment. This represents your typical standard two stage implant (the crown is never considered as a stage of the implant. This accords to a relatively good degree for the replacement of all the missing anterior teeth in the mouth but is not at all in accord with the natural state for replacing the missing posterior teeth, where as was previously mentioned, the upper molars are typically tri-rooted and the lower molars are typically bi-rooted.
The upper and lower jaws are made up of a narrow strip of softer, spongy, alveolar bone sandwiched between two thin outer harder cortical plates of bone. In the posterior regions the entire width of the jawbones is typically only 5 to 7 millimeters thick. The average interdental (anterior-posterior length between the teeth) space remaining when a molar tooth is lost is 10 to 12 millimeters long. The vertical depth of alveolar bone present where the tooth was lost can be as little as 5 to 10 millimeters before one encounters either the maxillary sinus space (in the upper jaw) and the inferior alveolar nerve (in the lower jaw).
To allow for a proper volume or thickness of jaw bone between the implant and the adjacent teeth so as to allow for a proper blood supply and health of the bone between the implant and the adjacent teeth, it has been accepted in the dental field to maintain a minimum distance of 2 millimeters between the implant and the adjacent teeth on either side of the implant. As noted above, this means that the head (top portion) of the implant at the height of the crestal bone should not typically exceed a diameter of 6 to 8 millimeters in a mesio-distal dimension (the distance between the adjacent teeth where the missing tooth used to be), based on the formula: interdental space (space left by the missing tooth) minus 4 millimeters (2 millimeters on each side of the implant)=maximum diameter of implant head. In the particular case of the posterior teeth (molars) it is typically either 10−4=6, or 12−4=8. As mentioned above, the entire width of the jawbones is typically between 5 to 7 millimeters thick (referred to in the dental field as its Bucco-Lingual dimension) in the posterior area. This means that in order to stay within the confines of the jawbone and not puncture the outer cortical plates of the jawbone, the maximum dimension of the head of a standard implant which is round in cross-section should typically not exceed 6 millimeters in diameter.
This means that the target bone site for a dental implant is very limited and requires the practitioner who wishes to place dental implants to have acquired a high degree of skill level and clinical experience.
Dental implants are typically placed using the following two surgical techniques: 1. Delayed Implant technique: the unsalvageable tooth is extracted and the entire root socket(s) are allowed to heal with bone filling the void(s) over several months. Once this healing process has been completed, the practitioner opens the gum and drills into the bone to create the osteotomy (bone preparation) to allow for the insertion of the dental implant. 2. Immediate Extraction-Immediate Implant technique: At the same visit, or within a period of 4 weeks or fewer, the practitioner extracts the unsalvageable tooth and immediately inserts the dental implant into the root socket voids or using a drill modifies this root socket or drills a new hole and places the implant into it. In the case of a molar tooth extraction the practitioner is left with multiple proximal root socket voids in the jawbone (where the multiple natural roots used to be) and an oval or rhomboid distal void (where the root trunk used to be). An implant designed to closely resemble this negative shape would be seen advantageous, as it would require minimal or no drilling of the fresh extraction site in order to placed said implant into it (minimizing pain for the patient) and significantly reduce the time required for bone healing of the implant site and time to functional loading since less bone needs to grow around the implant in order to adapt to it (when the patient could actually chew solid food on the implant supported crown). However, until the present teachings, there has been a reluctance to pursue such an approach because of the limited implant designs available to the surgeon.
In an attempt to provide for a multi-rooted tooth form implant, WO Pat. App. No. 2006/082610 Aug. 2006, Cito. D'Ambrosio and Vinci, describes a “multiple-root” form dental implant design with a “head” component which it calls a “collar” and a “root screw” component which it calls a “fixture”. For the sake of clarity the terms “head component” and “root screw” or alternatively “bone attachment” component used by the present teachings for these components will be used to describe these same respective implant components.
Another multi-root implant form design to the above described application is described in U.S. Pat. No. 2003/0180686, September 2003, Simmons.
Both of these applications describe a design wherein the “root screw” components are by necessity of smaller diameter or girth than the bore holes of the “head” component as their entire length (except for the limiting head) need to be inserted through these bore holes so that their wider diameter head can rest on the inner surface of the circumferential lip of the bore hole (which acts as a limiting stop) in order to relate these two components to each other.
This is a significant drawback in the structural design of both these applications for the following reasons: As noted above, there are significant limitations on the maximum interdental (mesio-distal distance between the teeth) and bucco-lingual (width of the jawbone) dimensions of the implant site. The diameter of the “head component” that can typically be accommodated in this limited implant site for missing molar teeth without puncturing this three-dimensional volume of the bone in both of the above two dimensions is itself quite limited. Therefore, the diameter of the bore holes contained within said head component must of necessity be of smaller diameter than the head component which contains them.
Both applications described above do not allow for the tight securing to each other of their head component and their root screw component at the time of initial insertion of these implant components into the fresh osteotomy of the jawbones (initial implantation). This is a significant drawback as it allows for potential micro-infiltration of pathogenic bacteria (at the time of initial implant surgery when the jawbone is directly exposed to the bacteria-laden oral environment as well as during the early stages of healing of the fresh osteotomy) into the micro-gaps between these unsecured endosseous (in bone) implant components and the creation of a reservoir of these pathogenic bone-resorbing bacteria between them. Additionally, as these endosseous (in the bone) implant components (the head and root screw components) are not tightly secured to each other, these components are free to shift their positions relative to each other during the several months that is required for the bone remodeling that occurs as part of the natural healing process of the osteotomy (implant preparation in the jawbone), another major drawback.
The above elements described may be critical requirements, as noted above, for the successful implantation of any dental implant and actually may be more critical requirements for the successful placement by the dental practitioner and long term viability of a “multi-rooted” posterior (molar) dental implant due to the larger number of components (compared to a “single-rooted” anterior implant) which must accurately be related to each other and related to the bone preparation fashioned to receive them. Additionally, a posterior molar implant should be able to handle the significantly greater amount of load (stress forces) it must withstand (typically 500 Newtons of force compared to 200 Newtons of force for the anterior teeth) due to its position and normal function requirements (holding up the bite and chewing forces) therefore the dimensional size of these implant components and their ability to withstand these significant force loads over decades is critical to the long term success or failure of these implant components and the assembled implant in its entirety.